1. Field of the Invention
The present invention relates to a method of manufacturing a self-support optical cable having superior appearance and a predetermined stable excess length.
2. Description of the Related Art
Conventionally, in a self-support optical cable, a support wire and an optical cable body have been integrated with each other. When such a cable is strung aerially, the opposite ends of the support wire are fixed to poles to hold the optical cable body portion. This cable has an advantage of good workability such that it is not necessary to string the support wire independently and the working of stringing can be finished at a time because the support wire and the optical cable body are integrated with each other. In the state where the self-support optical cable has been strung, however, the tension given to the support wire reaches several hundred kgf, and its extension percentage reaches about 0.2%. If such an extension of about 0.2% is always given to an optical fiber, a serious problem for durability is a matter of concern from the viewpoint of a long-time reliability.
A self-support optical cable which can solve this problem is shown in FIGS. 12 and 13. In FIGS. 12 and 13, the reference numeral 3 represents a support wire; 4, an optical cable body; 13, a common sheath; 13a, a neck portion; and 13b, a slit. In this self-support optical cable, the support wire 3 and the optical cable body 4 are coated with the common sheath 13 integrally. Normally, the outer diameter of the optical fiber body 4 is larger than the outer diameter of the support wire 3. Therefore, the common sheath 13 is formed into an eight figure shape (otherwise referred to as a figure eight shape). The slit 13b is formed intermittently in the neck portion 13a of this eight figure sheath. In FIG. 12, the self-support optical cable is strung and extended by tension given to the support wire 3 so that the optical cable body 4 is arranged in parallel with the support wire 3. In the case where no tension is given to the support wire 3, the optical cable body 4 has an excess length relative to the support wire 3, so that the optical cable body 4 is meandering when viewed from below as shown in FIG. 13.
A method of manufacturing such a self-support optical cable provided with an eight figure sheath is disclosed in Unexamined Japanese Patent Publication (kokai) No. Hei-7-230028. The manufacturing method disclosed in this publication is a method comprising the step of extruding a common sheath of a support wire and an optical cable body into an eight figure shape simultaneously, and the step of cooling the self-support optical cable with the softened common sheath while winding the self-support optical cable around a capstan, immediately after extruding. The outer circumferential surface of the capstan is cylindrical, and straight in parallel with its central axis. Since the self-support optical cable is wound on the cylindrical surface, there arises a difference between the circumference of the central axis of the support wire and the circumference of the central axis of the optical cable body so that an excess length of the optical cable body is obtained by this difference.
In addition, Unexamined Japanese Patent Publication (kokai) No. Hei-8-75969 discloses a similar manufacturing method using a grooved capstan. In this grooved capstan, a groove into which a support wire and an optical cable body can be inserted is formed in the outer circumferential surface of the capstan.
In such a conventional technique, a self-support optical cable with a softened common sheath was wound around a single capstan by one or several turns in the same direction. Accordingly, there was a problem in the cooled self-support optical cable that a bending might arise in a neck portion, so that a stable excess length could not be formed and the appearance became poor.
That is, in the manufacturing method disclosed in Unexamined Japanese Patent Publication (kokai) No. Hei-7-230028, the self-support optical cable with the softened common sheath is wound around the straight outer circumferential surface of the capstan and contacts therewith. Accordingly, as shown in FIGS. 14A, 14B and 14C, the shape of the neck portion 13a of the common sheath 13 is not only inconsistent but also shaped variously. In the manufacturing method disclosed in Unexamined Japanese Patent Publication (kokai) No. Hei-8-75969, as shown in FIG. 15A, the grooved capstan 6 is used. Therefore, the shape of the neck portion 13a of the common sheath 13 is restricted to some extent, so that the neck portion 13a can be prevented from bending inward as shown in FIG. 15B. However, it is impossible to prevent the neck portion 13a from bending outward as shown in FIG. 15C.
In addition, the excess length of the optical cable body with respect to the support wire in the self-support optical cable is changed according to the cable outer diameter, the deformation of the cable, the deflection amount of the cable, the hardness of the optical cable body, the temperature of cooling water, the manufacture line velocity, and the like. However, in order to confirm the excess length of the optical cable body, the condition of the optical cable with respect to the expansion of the support should be observed by actually laying the self-support optical cable, or the finished self-support optical cable should be divided to the support wire and the optical cable body so as to measure respective lengths.
A method as described in Unexamined Japanese Utility Model Publication No. Sho-62-81913 or Unexamined Japanese Patent Publication (kokai) No. Hei-2-156213, in which a tension is added to a support wire to give expansion and a common sheath covers the support wire and an optical cable body to which expansion is not applied, has the following problem. In case of using a tension-resistance body for the supporting wire in which seven steel wires having diameter of 2 mm are stranded, in order to obtain 0.3% excess length of the optical fiber body, the excess length is calculated by the formula of: (excess length)={(section of tension-resistance body).times.(Young's Modulus of tension-resistance body)}.times.100. Accordingly, it is calculated: 0.3%={1188 kg/(.pi..times.(2.0 mm/2).sup.2 .times.7.times.18000 kg/mm.sup.2)}.times.100. Thus, the tension of 1200 kg is necessary in a manufacturing facility. Since the maximum tension of a normal manufacturing facility is about 500 kg, a strong facility should be needed and there is a safety problem.
In a method in which the excess length is obtained by winding the support wire and the optical cable body around a guide wheel, if the outer diameter of the optical cable body is changed, the excess length of the support wire is also changed. In order to set the variation of the excess length within a predetermined range, a plurality of guide wheel units should be prepared for the variation ranges of the outer diameter of the optical cable body. For example, if the target value of the outer diameter of the optical cable body is set to "D", when the variation amount of the outer diameter of the optical cable body is .+-.10%, that is, the outer diameter of the optical cable body is D.+-.10%, the excess length can not be obtained in a predetermined length. Accordingly, the outer diameter of the optical cable body should be suppressed within the range of D.+-.3%. If D is 15 mm and the outer diameter of the guide wheel is 1000 mm, the variation of the outer diameter should be suppressed so that the outer diameter of the optical cable body is within the range of 15.+-.0.5 mm.
Furthermore, there is another problem that a desired excess length can not be obtained if the optical cable body slips on the guide wheel because of a large tension caused by a large supply drum of the optical cable body feeding machine. On the other hand, if the tension of the optical fiber cable body is large at the guide wheel, the sheath of the optical cable body may be flattened. Such flattening of the sheath causes various problems including a change in the winding diameter of the optical cable around the guide wheel, which results in a failure to obtain a predetermined excess length. If the tension of the optical cable body is changed, the expansion amount of the optical cable body itself is changed, thereby generating the deviation of the excess length.
In a method to manufacture the self-support optical cable in which a metal tape such as aluminum or stainless is attached longitudinally on the optical cable body, the metal tape is cyrindrially formed, and a common sheath is applied thereon, the tension generated by the formation when the metal tape is longitudinally attached to the optical cable body is applied to the optical cable body. Accordingly, the tension given to the optical cable body is very high, and the tension when passing through the guide wheel is also made large. Therefore, it is not possible to obtain a desirable excess length because of the same reason as described above.